The present invention relates to robots and more particularly to current sensing systems for robot arm power amplifiers used to energize the robot joint motors.
Power amplifiers are employed in robot arm control systems to supply the motor current needed to operate the axis drives and move the arm in accordance with a robot control program. Power amplifiers have been significant cost elements in the overall cost of robot control systems and it is thus important that power amplifiers be selected or designed to achieve quality performance with low cost.
Typically, power amplifier blocks have included internal circuitry that provides motor current control through on/off switching of switching devices within the power block. While this approach has been needed to coordinate power amplifiers with the position or position/velocity control loop configurations employed in controllers for numerical machine tools and robot arms, it has also made power amplifiers costly and has highly limited the external controllability of the internal power amplifier switching devices.
Nonetheless, the present and foreseeable volume of robot control systems does not justify the development of more suitable internally controlled power amplifier packages by any single manufacturer. To achieve desired performance in the field of robot control, it is thus necessary to look to the use of less costly commercially available power devices.
Hybrid power devices or blocks have recently been developed as low cost devices for high-volume nonservoed applications. The hybrid power blocks include power semiconductor and clamp diodes in multiple legs of a power amplifier bridge in a single package suitable for direct mounting on a heat sink. No control circuitry is included in the hybrid power block, and the blocks thus are characterized with control flexibility as well as low cost.
Power amplifiers for high power robotic drives typically employ pulse width modulation control schemes. The power switch elements are usually configured in an H bridge where brush type DC joint motors are used.
A new and improved digital robot control system described in the cross-referenced patent applications advances the state of the robot control art through a completely digital implementation of robot position, velocity and current control loops. In achieving completely digital control, an improved PWM control scheme described fully in patent application W.E. 53,225 enables use of the low cost hybrid power blocks.
The new robot control system provides control over the torque developed by each of the axis drive motors. Since the torque developed by a motor is proportional to the current flowing through its windings, such an advanced robot control system requires measurement of the actual currents flowing through the axis motors to provide time stabilized current sampling synchronized to the sampling frequency of position and velocity control loops in the robot control system.
Normally, current sensing could be accomplished through insertion of a current sense resistor in series with each leg of the power amplifier bridge. The voltages across the sense resistors would provide a measure of the axis motor current. However, insertion of a series resistor in each bridge leg is not feasible since the noted commercially available hybrid power blocks only have external leads which operate as common voltage and return or power supply rail leads. Moreover, analysis indicates that, even if such a leg resistor arrangement is employed with the low cost hybrid power block, circulating currents can flow through the motor without flowing through the power supply leads thereby making the accuracy of sensed current values uncertain.
The present invention is directed to a current sensing system which provides for sensing motor current while making use of the low cost hybrid power blocks for a robot control especially for a digital robot control.